Because of their high speeds and direct bandgap capabilities, gallium arsenide devices offer considerable promise for use as high speed electronic and photonic devices and integrated circuits. Such devices typically comprise a plurality of layers of gallium arsenide and aluminum gallium arsenide which are doped to n or p type conductivity and configured to operate as semiconductor electronic devices such as bipolar or field effect transistors or as photonic devices such as photodetectors or surface emitting lasers.
A preferred method of fabricating a gallium arsenide layered structure is molecular beam epitaxy such as the process of metalorganic molecular beam epitaxy (MOMBE) described by G. J. Davies et al, 3 Chemtronics 3 (1988). Other molecular beam processes are described by W. T. Tsang, Journal of Electronic Materials, p. 235 (1986).
In essence a molecular beam process involves disposing a substrate within a low pressure growth chamber, heating the substrate and directing onto the substrate a molecular beam of gaseous molecules which decompose to form a desired layer. Processes referred to as MBE typically use only elemental sources for the Group III, Group V, and dopant elements, whereas MOMBE processes use a wide variety of elemental and compound gaseous sources providing at least one of the Group III or dopant elements. Typical MOMBE processes used in the fabrication of pnp and npn transistors are described in the copending application and patent of C. R. Abernathy et al, Ser. No. 07/662549 and U.S. Pat. No. 5,171,704 both filed Feb. 28, 1991, and incorporated herein by reference.
One undesirable feature of prior art molecular beam processes is their use of arsine (AsH.sub.3) or arsenic as a source in the growth of gallium arsenide. A number of device fabrication schemes require selective regrowth of gallium arsenide layers in unmasked limited areas of masked substrates. At desired low temperatures (.ltoreq.600.degree. C.) regrowth using arsine or arsenic is non-selective, leading to growth not only on the unmasked substrate but also upon the mask.
Important classes of semiconductor devices require selective growth of high purity and n-type gallium arsenide layers at low temperatures below 600.degree. C. in order to avoid deterioration of previously grown layers. Such selective growth cannot be readily achieved using conventional methods. Selective growth has been achieved using triethylgallium (TEG) as the gallium source, but TEG requires temperatures above 600.degree. C. for selectivity. Selective growth at temperatures below 600.degree. C. has been achieved using trimethylgallium (TMG) as the gallium source, but TMG introduces high levels of carbon which are not desirable in high purity or n-type layers. Accordingly, there is a need for improved methods for selectively growing gallium-containing layers.